AVES. 



285 



a state suitable for digestiun. The gullet, after leaving the crop, 

 shortly opens into a second cavitj^ called the " proventricubis," 

 which is the true digesting stomach, and is richly supplied with 

 glands which secrete the digestive fluid or gastric juice (p). This, 

 in turn, opens into a muscular cavity which is called the " gizzai'd " 

 (r/), and which leads into the commencement of the small intestine. 

 The characters of the gizzard vary with the nature of the food. In 

 the Birds of Prey, which live on an easily digested animal diet, the 

 walls of the gizzard are thin and membranous. In the grain-eating 



Fig. 205. — Digest.i\'e system of the common Fowl (iifter Owen), o Gullet; c Crop; 

 J) ProveiitricnluM ; g Gizzard ; sm Small intestine ; /j Intestinal cjeca ; I Large 

 intestine ; d Cloaca. 



Birds, such as the Fowls, whose bard food requires to be crushed 

 before it can be properly digested, the walls of the gizzai'd are 

 extremely thick and muscular, and the inner lining is hard and 

 horny. In these Birds the gizzard constitutes a kind of grinding 

 apparatus, like the stones of a mill; wliilst the "crop" may lie 

 compared to the " hopper " of the mill, since it sup]i]ies to the 

 gizzard " small successive quantities of fooil as it is wanted " ((Jwen). 

 The grinding action of the gizzard is further assisted by the small 

 pebbles and gravel which, as is well known, so many Ijirds are in 

 the habit of swallowing. These pebbles take the place of teeth, 



